Integrated circuitry fabrication may involve lithographic processing to transfer patterns formed in an imaging layer to underlying substrate material which will form part of the finished circuitry. For example, an imaging layer such as photoresist is provided over a layer to be patterned by etching. The imaging layer is then photolithographically processed such that selected regions of the imaging layer are exposed to suitable conditions which impact the solvent solubility of the exposed regions versus the unexposed regions.
The photolithographic processing may comprise subjecting the imaging layer to actinic energy passed through a mask pattern. The mask pattern has windows therethrough. Some regions of the imaging layer are exposed to the actinic energy passed through the windows, and other regions of the imaging layer are shadowed by non-windowed regions of the mask so that such other regions are not exposed to the actinic radiation (or at least are exposed to less actinic energy than the fully exposed regions). The imaging layer is then solvent processed to remove one or the other of the exposed or the unexposed regions, thereby forming the imaging layer to have mask openings extending partially or wholly therethrough to the underlying layer being patterned. In one type of processing, the substrate is then subjected to a suitable etching chemistry which is selected to etch the underlying layer or layers at least at a greater degree than the imaging layer, thereby transferring the imaging pattern to the underlying circuitry layer or layers. Alternate to etching, the substrate may be ion implanted or otherwise processed through the mask openings in the imaging layer.
In the past, some types of photolithographic patterning tools have been referred to as photomasks, and others have been referred to as reticles. The term “photomask” has been used to refer to tools which define a pattern for an entirety of a wafer, and the term “reticle” has been used to refer to tools which define a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized interchangeably to refer to radiation-patterning tools that encompass either a portion of a wafer or an entirety of a wafer.
Various types of photomasks are known in the art. For example, one type of mask includes a transparent plate covered with regions of a radiation blocking material, such as chromium, which is used to define the semiconductor feature pattern to be projected by the mask. Such masks are called binary masks, since radiation is completely blocked by the radiation blocking material and fully transmitted through the transparent plate in areas not covered by the radiation blocking material. Accordingly, such use binary features within the mask patterning area which include an opaque layer to essentially completely block the transmission of the actinic energy.
Due in part to limitations imposed by the wavelength of light or other actinic energy used to transfer the pattern, resolution can degrade at the edges of the patterns of binary photomasks. Such has led to the development of phase-shifting photomasks which can increase the resolution of patterns by creating phase-shifting regions in transparent areas of the photomask. Standard phase-shifting photomasks are generally formed in one of two manners. In a first, transparent films of appropriate thickness are deposited and patterned over the desired transparent areas using a second level lithography and etch technique. In a second, vertical trenches are etched into a transparent substrate. In both instances, the edges between the phase-shifted and unshifted regions generally result in a transition between high and low refractive index regions. These types of masks include transmission areas on either side of a patterned opaque feature. One of these transmission areas transmits light 180° out of phase from the other transmission areas, and both sides transmit approximately 100% of the incident radiation. Light diffracted underneath the opaque regions from the phase-shifted regions thus cancels each other, thereby creating a null or “dark area”.
Another type of phase-shifting mask is known as an “attenuated” or “half-tone” phase shift mask. Such masks include both transparent and less transmissive regions. Actinic energy/radiation passing through a partially transmissive region of such a mask generally lacks the energy to substantially affect a resist layer exposed by the mask. Moreover, the partially transmissive regions of such masks are designed to shift passing radiation 180° relative to the radiation passing through the completely transmissive regions and, as a consequence, the radiation passing through the partially transmissive regions destructively interferes with radiation diffracting out from the edges of the completely transmissive regions. Masks have been proposed that use both binary features and attenuating phase-shift mask features in the device area.
As minimum device pitch falls below 100 nanometers (i.e., where minimum feature size or minimum critical dimension falls below 50 nanometers), attenuated phase-shift photomasks may begin to loose contrast with specific wavelengths of actinic energy.